Small Molecules, Energy, and Biosynthesis

The Chemical Components of a Cell

Outline Cell Chemistry Is Based on Carbon Compounds Cells Use Four Basic Types of Small Molecules Sugars Are Food Molecules of the Cell Fatty Acids Are Components of Cell Membranes Amino Acids Are the Subunits of Proteins Nucleotides Are the Subunits of DNA and RNA Summary Figures Figure 2-3: The structure of the monosaccharide glucose, a common hexose sugar Figure 2-4: Eleven disaccharides consisting of two delta-glucose units Panels Panel 2-3: An outline of some of the types of sugars commonly found in cells References Sharon, N.Carbohydrates. Sci. Am. 243(5):90-116, 1980 [PubMed]

Sugars Are Food Molecules of the Cell3     The simplest sugars - the monosaccharides - are compounds with the general formula (CH2O)n, where n is an integer from 3 through 7. Glucose, for example, has the formula C6H12O6 (Figure 2-3). As shown in Figure 2-3, sugars can exist in either a ring or an open-chain form. In their open-chain form sugars contain a number of hydroxyl groups and either one aldehyde (H>C=O) or one ketone (>C=O) group. The aldehyde or ketone group plays a special role. First, it can react with a hydroxyl group in the same molecule to convert the molecule into a ring; in the ring form the carbon of the original aldehyde or ketone group can be recognized as the only one that is bonded to two oxygens. Second, once the ring is formed, this carbon can become further linked to one of the carbons bearing a hydroxyl group on another sugar molecule, creating a disaccharide (Panel 2-3, pp. 52-53). The addition of more monosaccharides in the same way results in oligosaccharides of increasing length (trisaccharides, tetrasaccharides, and so on) up to very large polysaccharide molecules with thousands of monosaccharide units. Because each monosaccharide has several free hydroxyl groups that can form a link to another monosaccharide (or to some other compound), the number of possible polysaccharide structures is extremely large. Even a simple disaccharide consisting of two glucose residues can exist in eleven different varieties (Figure 2-4), while three different hexoses (C6H12O6) can join together to make several thousand different trisaccharides. It is very difficult to determine the structure of any particular polysaccharide because one needs to determine the sites of linkage between each sugar unit and its neighbors. With present methods, for instance, it takes longer to determine the arrangement of half a dozen linked sugars (those in a glycoprotein, for example) than to determine the nucleotide sequence of a DNA molecule containing many thousands of nucleotides (where each unit is joined to the next in exactly the same way).     Glucose is the principal food compound of many cells. A series of oxidative reactions (see p. 62) leads from this hexose to various smaller sugar derivatives and eventually to CO2 and H2O. The net result can be written     C6H12O6 + 6O2 -> 6CO2 + 6H2O + energy     In the course of glucose breakdown, energy and "reducing power," both of which are essential in biosynthetic reactions, are salvaged and stored, mainly in the form of ATP in the case of energy and NADH for reducing power. We discuss the structures and functions of these two crucial molecules later in the chapter.     Simple polysaccharides composed only of glucose residues - principally glycogen in animal cells and starch in plant cells - are used to store energy for future use. But sugars have functions in addition to the production and storage of energy. Important extracellular structural materials (such as cellulose) are composed of simple polysaccharides, and smaller but more complex chains of sugar molecules are often covalently linked to proteins in glycoproteins and to lipids in glycolipids.

© 1994 by Bruce Alberts, Dennis Bray, Julian Lewis, Martin Raff, Keith Roberts, and James D. Watson.